In the game, you have both Fatigue and Dreamself Fatigue. If you are unawakened, you do not have to worry about Dreamself Fatigue. Fatigue represents how tired your character is.
Fatigue from Strife[]
Fatigue grows the more you strife, and you can view how high your fatigue is at the bottom of a page. It can stack up more and more, but upon reaching 1000 fatigue, your character's Offense/Defense will grow a Fatigue Penalty, which will appear at the bottom of a page as a percentage. The percentage starts off low, but the higher it reaches from strifing, it will begin to decrease your Offense/Defense. Upon reaching 1500 fatigue, around which Fatigue Penalty reaches around 50%, your Offense/Defense bar will drop to negative numbers, thus making even an Imp or Carapace Ant impossible to defeat. The only way to recover Fatigue and your Offense/Defense is to switch to your Dreamself/Waking Self or to simply do other activities rather than dungeons and strifes.
Having a Dreamself to Strife is a good advantage when you are trying to reach the top of the echeladder. Once the Waking Self/Dreamself's fatigue is too high, you can switch to the other and continue strifing there, thus not only reaching you higher the echeladder, but also recovering your other self's fatigue.